No Stupid Questions

Arsenic is a classic murder poison. It's been known since anciemt times, though possibly unsuited to your onset requirement. Acute poisoning by ingestion is generally within a few hours, but if your character sustains lower doses over time, you could probably draw out the timeline to whatever you wanted. It would be obvious that the character is unwell during this time, but the symptoms aren't super specific and could be confused with e.g. food poisoning.

Or just invent a mushroom like others said. The toxins are diverse enough that I doubt anyone would be too upset if you tuned it exactly to your timeline and desired symptoms.

Depending on your setting and desired outcome for the poisoner, uraninite (aka pitchblende) might be an option. It has historical uses in glass making and pottery glazing, which could provide justification for why someone would have it.

It contains Uranium, which is radioactive, but I don't believe will bioaccumulate, but can build up on surfaces, tools, and clothing providing a source of long-term radiation exposure. In addition, it contains lead, which does bioaccumulate, providing a source of gradual long term poisoning as well as radium which also bioaccumulates and is radioactive, providing an additional source of longterm radiation exposure.

How slow are you talking? Days? Weeks? months?

Go ask chatgpt or something similar for old medical herbs used back in the day that might also posion you. I inspire to write a book one day, and ive found ai to be really informative. The best part is, even if they're wrong its going in a fiction story(i hope) where it wouldn't matter.

Mushrooms are a good option, and you can just make up a species if you want specific time frames/symptoms. Mushrooms can cause a lot of weird symptoms.

There's also a brain eating amoeba or other sickness from still water (people back in the day were very aware of tainted water).

If you have access to polar animals, a unique poisoning would be vitamin A toxicity from their livers. It's a horrific way to die, though (skin sloughing off).

Apple seeds contain cyanide. You'd have to crush and eat anywhere from 150 to a few thousand seeds for it to be fatal though. I'm sure that hasn't stopped authors from using it before.

• Heavy metals: lead, mercury
• Arsenic in small doses over a long period

I don’t know of any plants, but I do know that the leaves of nightshades (potato, tomato, eggplant, capsicum, tobacco) are ~~poisonous~~ toxic in large doses.

Alcohol. Sometimes it takes 30 or 40 years to be effective. Not very good for murder, but wildly popular for suicide.

If you want long-term, look to the liver. Alcohol will destroy it over time, but so will viral hepatitis. Have you considered slow acting diseases like hepatitis or HIV? Or something intensely carcinogenic?

I’m neither a writer nor a scientist, but there’s a copy of this on my bookshelf and I wish I could lend it to you: Deadly Doses: The Writer’s Guide to Poisons

In almost every case in fictional writing it's better to make up a poison then use a real one. That way you don't have someone picking it apart later. Also you can give it whatever properties you want/need. Now excuse me while I continue to work on my immunity to iocane powder.

Whole maybe not “poisons” by definition I have a couple scary stories of people working in damp, moldy office and basement environments and after a couple years getting rare autoimmune and neurological disorders that killed them. One being my uncle, my family tried to get his workplace to test where he worked because the doctors said that’s most likely where he contracted it, but they refused. We weren’t looking for money, just trying to save the next guy. I was fairly young when this happened so I don’t remember all the details.

I'm thinking metal poisoning over time. Lead or copper, for example.

Apple seeds contain cyanide, simply crushing them will release the poison and it can be added to something else, although it's not very slow.

There's also the Destroying Angel/Death Cap mushroom, whose symptoms can take up to a day to even start, by which point the toxins have been incorporated and destruction of liver and kidney tissue is irreversible. They also contain toxins that can cause severe DNA damage, making it so your body can no longer repair itself after exposure, and you slowly die cell by cell.

What do you mean by slow? Time till symptoms? time till death? Hours? Days? Weeks?

Raw castor beans contain ricin its a cool looking plant that gets big. My neighbor grew on once accidentally.

symptoms commonly begin within two to four hours, but may be delayed by up to 36 hours.

Unless treated, death can be expected to occur within 3–5 days; however, in most cases a full recovery can be made.

Actually a lot of beans are toxic when raw, but not deadly. Raw lima beans are special though; they contain something that the human body breaks down into cyanide. No clue how long that takes or how many it would take to cause harm.

Heavy metals (mercury, arsenic, cadmium, lead) are known for building up over time with many exposures. Think mad hatter syndrome, etc. but exposures can also be acute if high enough. IIRC acute arsenic poisoning makes you vomit and diarrhea until you die of dehydration after days or weeks.